This invention relates to amplifiers and amplifier circuits used in tuners, and more specifically to variable gain low noise amplifiers.
U.S. Pat. 5,737,035 dated Apr. 7, 1998, shows a tuner circuit. The front end of such a tuner requires a broadband, highly linear Variable-gain Low Noise Amplifier (VLNA).
The VLNA""s input comes from either an antenna for wireless broadcasts or from a coaxial cable for cable transmission. The output of the VLNA supplies the input of the first up-converting mixer. The noise figure specification for the VLNA is highly critical, and has the highest impact on the overall noise figure of the system. Non-linearities of the amplifier also have a large effect on the proper operation of the tuner.
LNA""s are typically used to meet the narrow-band requirements of cellular systems.
However, a television tuner must generally receive carriers from 50 MHZ to over 860 MHz. A narrow bandwidth system also typically has less stringent linearity specifications because fewer intermodulation distortion products fall in-band. Finally, because the incoming signal power to a tuner may vary by many orders of magnitude, an LNA must have a continuously-variable gain. This gain variability function adds noise, distortion, and complexity to the LNA.
FIG. 1A shows an LNA. In its basic form, the differential transistors, 1Q1 and 1Q2 are supplied with DC voltage through 1 Vcc and biased with 1Vb and resistors 1Rb. The variable collector and emitter resistors, 1Rc and 1Re are used to control the gain of the amplifier. At low frequencies, the gain of the FIG. 1A amplifier is generally set by the formula:                               1          ⁢          Rc                          1          ⁢                      xe2x80x83                    ⁢          Re                                    (        1        )            
However, at high frequencies, inherent parasitic capacitances usually arise in many of the amplifier components which limit its frequency response.
These capacitances are inherent in the devices themselves. They arise generally due to the operating characteristics of semiconductor materials. Therefore, little can be done to change their existence. FIG. 1B shows a figurative representation of the device-specific high-frequency capacitances. In the bipolar transistors shown in FIG. 1B, capacitances typically arise between the collector and base terminals, 1 Cxcexc. Capacitances also usually arise in the variable resistors, 1Cc and 1Ce. Therefore, capacitors would typically shunt both the collector and emitter resistors. A capacitor across 1Rc will generally limit the frequency response, which essentially creates a low pass filter on the output signal. Conversely, a capacitor across 1Re will generally increase the overall gain of the amplifier in addition to increasing the frequency response, which essentially creates a high pass filter on the output signal. The combined effect generally limits the bandwidth of the amplifier. However, an ideal VLNA should preferably have a flat response without a high- or low-pass filtering effect.
As referenced above, a VLNA may be used in tuner applications to amplify the incorning channel signal. Therefore, it may have to amplify up to 133 different channel signals in a linear manner. With so many signals entering the amplifier, each channel could generally interact with other channel frequencies creating intermodulation distortion and harmonics. For this reason, narrow bandwidth VLNAs typically have less stringent linearity requirements than wide-bandwidth LNAs.
Another method generally used to avoid the intermodulation distortion and harmonics is to place an inductor-capacitor (LC) tracking filter on the front-end of the tuner. The LC tracking filter is typically tuned to allow fewer channels into the remainder of the tuner. Allowing fewer channels into the tuner generally reduces the chances for intermodulation and relaxes the linearity requirements for the rest of the amplifier circuit. With this method, the LC tracking filter must usually be highly selective, meaning that it must precisely filter to within a small frequency range. It must also be generally able to tune its center frequency over a wide frequency range. In order to accomplish these requirements, large inductors and extensive circuitry must typically be used. However, because inductors are not easily fabricated using integrated circuit technology, selective LC filters are generally not well-suited for integrated circuit (IC) applications. Large, high-quality inductors and capacitors are also more expensive and substantially larger than devices typically fabricated on IC substrates, thereby making the addition of such elements a large percentage of the manufacturing expense and undesirably increasing the size requirement of the entire circuit application.
Considering the problems inherent to the current state of VLNAs, it would be advantageous to have a variable gain low noise amplifier which has a wide and variable bandwidth with good high frequency response, good linear amplification, and suited for fabrication substantially on a single integrated circuit substrate.
These and other features and technical advantages are achieved by a system and method which increases the bandwidth and high frequency response of a VLNA by dividing the amplifier into at least two lower-gain amplification stages and adding a linearly-variable capacitance to the second stage which essentially compensates for the spectral effect of the circuit""s inherent parasitic capacitances. The system and method also provides a mechanism to maintain high input or output linearity relative to the type of signals being amplified.
To reduce the effective capacitance on the amplifier, it is preferably divided into at least two stages, each of which has a lower gain than an equivalent single stage VLNA. In a two stage device, variable common terminal resistors are preferably added to the first stage, while variable load resistors are added to the output stage. All of the remaining resistors in the VLNA are constant value components. The output stage also preferably includes a variable capacitance which compensates for the inherent capacitance of the amplifier circuit elements. By implementing the reduced gain of each individual amplification stage, reducing the total number of variable resistors per stage, and adding the variable capacitance, the bandlimiting effect of the overall circuit capacitance is reduced, thus increasing, or at least maintaining, the amplifier""s overall bandwidth and improving the high frequency response.
The present invention also maintains high application-specific linearity through its method of varying the gain of the amplifier. For signals requiring a high input linearity, the amplifier adjusts gain by varying the load resistors of the output stage, while keeping the common terminal resistors of the input stage constant. Conversely, for signals requiring high output linearity, the amplifier adjusts gain by varying the common terminal resistors of the input stage, while keeping the load resistors of the output stage constant.
In order to maintain linearity of the varied resistance and capacitance of the circuit, the invention preferably provides for a network of resistors coupled to resistor-associated MOSFET transistors such that successively varying the control voltages for each resistor-related MOSFET device varies the effective resistance of the device, thus, adding or subtracting resistance to the circuit path. This method and system for varying resistance results in a linear and predictable variation. Similarly, the invention preferably provides a network of capacitors coupled to capacitor-related MOSFET transistors such that switching on the connected capacitor-associated MOSFET devices adds the capacitance of the associated capacitor to the circuit path. As with the variable resistor configuration, as the control voltages move the MOSFETs between on and off states, the varied effective resistance in the MOSFET triode region of operation preferably varies the amount of capacitance added to or subtracted from the circuit. Varying the capacitance in this preferred manner provides linear and predictable changes in circuit capacitance.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.